diff --git a/Makefile b/Makefile
index 1ef9d45..a9449e4 100644
--- a/Makefile
+++ b/Makefile
@@ -6,8 +6,9 @@ default: F3DEX3_BrZ F3DEX3_BrW
 
 # List of all compile-time options supported by the microcode source.
 ALL_OPTIONS := \
-  CFG_G_BRANCH_W
-  
+  CFG_G_BRANCH_W \
+  CFG_DEBUG_NORMALS
+
 ARMIPS ?= armips
 PARENT_OUTPUT_DIR ?= ./build
 ifeq ($(PARENT_OUTPUT_DIR),.)
diff --git a/README.md b/README.md
index e0f0952..b8858e1 100644
--- a/README.md
+++ b/README.md
@@ -122,7 +122,7 @@ For an OoT codebase, only a few minor changes are required to use F3DEX3.
 However, more changes are recommended to increase performance and enable new
 features.
 
-There is only one build-time option for F3DEX3: `make F3DEX3_BrW` if the
+Select the correct version of F3DEX3 for your game: `make F3DEX3_BrW` if the
 microcode is replacing F3DZEX (i.e. OoT or MM), otherwise `make F3DEX3_BrZ` if
 the microcode is replacing F3DEX2 or an earlier F3D version (i.e. SM64). This
 controls whether `SPBranchLessZ*` uses the vertex's W coordinate or screen Z
@@ -432,6 +432,36 @@ two tris, saving a substantial amount of DMEM.
   tris. In F3DEX2, the fog settings at vertex load time would always be used,
   even if they were changed before drawing tris.
 
+
+## Debugging
+
+To help debug lighting issues, add `CFG_DEBUG_NORMALS` to the `OPTIONS :=` line
+of your selected microcode version in the Makefile (near the bottom), then
+`make clean` and `make` again. This feature causes the vertex colors of any
+material with lighting enabled to be set to the transformed, normalized world
+space normals. The X, Y, and Z components map to R, G, and B, with each
+dimension's conceptual (-1.0 ... 1.0) range mapped to (0 ... 255). This also
+breaks vertex alpha and texgen / lookat.
+
+Some ways to use this for debugging are:
+- If the normals have obvious problems (e.g. flickering, or not changing
+  smoothly as the object rotates / animates), there is likely a problem with the
+  model space normals or the M matrix. Conversely, if there is a problem with
+  the standard lighting results (e.g. flickering) but the normals don't have
+  this problem, the problem is likely in the lighting data.
+- Check that the colors don't change based on the camera position, but DO change
+  as the object rotates, so that the same side of an object in world space is
+  always the same color.
+- Make a simple object like an octahedron or sphere, view it in game, and check
+  that the normals are correct. A normal pointing along +X would be
+  (1.0, 0.0, 0.0), meaning (255, 128, 128) or pink. A normal pointing along -X
+  would be (-1.0, 0.0, 0.0), meaning (0, 128, 128) or dark cyan. Bright, fully
+  saturated colors like green (0, 255, 0), yellow (255, 255, 0), or black should
+  never appear as these would correspond to impossibly long normals.
+- Make the same object (octahedron is easiest in this case) with vertex colors
+  which match what the normals should be, and compare them.
+
+
 ## Credits
 
 F3DEX3 modifications from F3DEX2 are by Sauraen and are dedicated to the public
diff --git a/build.sh b/build.sh
index 3a1fdb5..2e62cee 100644
--- a/build.sh
+++ b/build.sh
@@ -9,5 +9,5 @@ cp build/F3DEX3_BrW/F3DEX3_BrW.code ../../Mods/HackerOoT/data/F3DEX3_BrW.code
 cp build/F3DEX3_BrW/F3DEX3_BrW.data ../../Mods/HackerOoT/data/F3DEX3_BrW.data
 make -C ../../Mods/HackerOoT -j12
 #cp ../../Mods/HackerOoT/HackerOoT.z64 /media/`whoami`/SOME2/
-../../Flashcarts/SummerCart64/sw/deployer/target/debug/sc64deployer upload ../../Mods/HackerOoT/HackerOoT.z64
+../../Flashcarts/SummerCart64/sw/deployer/target/release/sc64deployer upload ../../Mods/HackerOoT/HackerOoT.z64
 printf "\n====\nDone\n====\n"
diff --git a/f3dex3.s b/f3dex3.s
index 4638a56..cc62ae9 100644
--- a/f3dex3.s
+++ b/f3dex3.s
@@ -2202,9 +2202,16 @@ lt_after_xfrm_normals:
     jal     lt_normalize
      luv    vPairLt, (ltBufOfs + 0)(curLight) // Total light level, init to ambient
     // Set up ambient occlusion: light *= (factor * (alpha - 1) + 1)
+.if CFG_DEBUG_NORMALS
+.warning "Debug normals visualization is enabled"
+    vmudh   $v29, vOne, $v31[5] // 0x4000; middle gray
+    j       vtx_return_from_lighting
+     vmacf  vPairRGBA, vNrmOut, $v31[5] // 0x4000; + 0.5 * normal
+.else
     vmudh   $v29, vOne, $v31[7] // Load accum mid with 0x7FFF (1 in s.15)
     vmadm   vCCC, vPairRGBA, $v30[0] // + (alpha - 1) * aoAmb factor; elems 3, 7
     vcopy   vPairNrml, vNrmOut
+.endif
     beqz    $12, lt_loop // Not specular or fresnel
      vmulf  vPairLt, vPairLt, vCCC[3h] // light color *= ambient factor
     // Get vNrmOut = normalize(camera - vertex), vAAA = (vPairNrml dot vNrmOut)